Conventional communications systems typically include two DCEs connected across a communication connection, such as a subscriber loop associated with a public-switched-telephone-network (PSTN). The communication connection is comprised of a pair of copper wires that are terminated at the DCE so that the DCEs may communicate with each other via either half-duplex or full-duplex communications. These DCEs normally include a voltage driver that has a low source impedance for driving the data signal across the connection. Consequently, to effect multipoint communication, bridging additional voltage-driven devices to an end of the communications connection significantly loads the connection making it difficult to successfully communicate across the connection,
As a result of the difficulty of bridging multiple voltage-driven DCE tributaries to the connection, in order to increase the number of DCEs at a particular location, prior art systems typically increase the number of communications connections such that each tributary at a customer premises is serviced by a separate communications connection. The insertion of additional connections into the system increases the overall cost of the system, particularly when the connection is a subscriber loop associated with a PSTN.
Recently, a multipoint system has been developed by the assignee of the present application that allows a plurality of tributaries to be bridged to a standard PSTN telephone twisted pair. The DCEs coupled to each of the tributaries communicate with a DCE located at the central office via a particular polling protocol. Each of the DCEs coupled to the tributaries comprises a current source-driven (i.e., driven by a high-impedance source) analog front end circuit that couples each DCE to its respective tributary via a dry transformer. In order to prevent the DCEs connected to the communications connection from pulling DC from the PSTN central office equipment, a DC blocking capacitor is inserted into the analog front end circuit of each DCE.
This DC blocking capacitor and the transformer of the analog front end circuit form an equivalent series RLC circuit. Multi-band communications over the multipoint system may include frequencies ranging from 15 Hz for the ring signal on the low end, from 200 Hz to 4000 Hz for voice-band modem communications and voice communications, and from approximately 20 KHz to 80 KHz at the upper end for data transmission. Practical values for the inductors of the transformer and for the DC blocking capacitor cause an impedance null to be created in or near the voice band with a low quality factor Q, usually resulting in a large band of attenuation in the voice band.
Accordingly, a need exists for a DCE that can be employed in a multipoint environment and which has an analog front end circuit designed to eliminate the impedance null that would otherwise occur in or near the voice band.